Herbal medicines have currently become popular as alternative medicines 1. Pharmacological activities of several plant species are interesting. Phyllanthus amarus Schum. & Thonn. and Phyllanthus emblica Linn., the plants in family Euphorbiaceae, have been found in many tropical countries including Thailand. Both plants are well-known as medicinal herb with various promising pharmacological activities 2. Their common activities include antioxidant 3,4; antimicrobial 5,6; hepatoprotective activity against ethanol 7,8, paracetamol 9,10 and carbon tetrachloride 11-13; anti carcinogenic activity against N-nitrosodiethylamine 14 etc. The toxicity data of both plants in animals were previously reported 15,16. However, the plant which has no toxicity in animal test may has herb-drug interaction 17,18. The information on drug interaction with P. amarus and P. emblica is required to assure safety of using the plants in human. Cytochrome P40 (CYP), the phase I enzyme system primarily in the liver, normally plays a key role in metabolism of most currently used medicines. Major CYPs in human liver responsible for xenobiotic and drug metabolism are in families 1, 2 and 3. These isoformsu include CYP1A2, CYP2A6, CYP2B6, CYP2C8/2C9, CYP2C18/2C19, CYP2D6, CYP2E1 and CYP3A4. Modulation of CYP either inhibition or induction has been shown to be one of the most important etiology of drug-drug interaction 19. Inhibition of CYPs can be advantageous in term of protection of human from toxicity/mutagenesis/carcinogenesis by many xenobiotics. Therefore, the aim of this study was to investigate the inhibitory effects of P. amarus and P. emblica aqueous extracts on CYP1A2, CYP2C9, CYP2D6, CYP2E1 and CYP3A4 so as to evaluate the herb-drug interaction potential as well as the advantageous property of these plant extracts against xenobiotic bio activation.